The software maker is gearing up for a big fall ad blitz for the 3-year-old operating system. The campaign is slated to start in September, with ads touting the security improvements of Windows XP Service Pack 2. As the security push starts to wind down in mid-October, Microsoft's marketing will focus on new technologies that build on Windows XP, including portable media centers and a new version of Windows Media Center.

With Longhorn still at least two years away, Microsoft faces the challenge of trying to spur sales without an all-new operating system. The company does have updated versions of its Tablet and Media Center editions, as well as a version of XP for 64-bit computers that will all ship later this year.

At the same time, a lot of PCs still run older software. Although Microsoft noted earlier this week that it has sold 210 million copies of Windows XP, corporate vice president Tom Button said Tuesday that older "Windows 9x" operating systems such as Windows 98 still serve more than half of the PCs in use.

"We know there is a great opportunity for us to move people off of 9x and onto Windows XP," Button said. Only a small portion of computers running the older operating systems can be upgraded to Windows XP, meaning that most of the battle is focused on convincing people to buy a new PC.

That's where the ad campaign will come in. Microsoft, which has not had a broad push for Windows since the launch of XP in 2001, will try to make a good case for the benefits of buying a new PC.

For a time, Microsoft considered other options to try to push Windows XP, including looking at an update to the operating system itself, but instead decided to focus on a marketing effort.

The company had already shifted considerable resources toward Windows XP Service Pack 2. Typically, Microsoft's service packs are mostly focused on bug fixes, but with Service Pack 2, Microsoft has tried to make the operating system significantly more secure, adding a beefed-up firewall, among other features.